you've probably long since sorted this but I spotted the available answers and thought I'd post anyway.
By Default Rainmeter positions are set to normal (like any other window)
If you right-click on your gadget --> click Setting --> Position--> You should see half a dozen options ranging from Always on Top to On Desktop.
Click On Desktop then when you press Show Desktop it wont minimise the rainmeter gadgets
If you have a lot of gadgets that need placing on the desktop then right click any of your gadgets -->Rainmeter Menu --> Edit Settings... (which should bring up a text file)
Find&Replace 'AlwaysOnTop=0' with 'AlwaysOnTop=-2' and click Replace All
Hope this helps.